In modern wireless systems, such as for example cellular wireless and fixed radio access wireless networks, there is a need for equipment operating with radio frequency signals, such as radio transceiver equipment in user equipment or at base stations or access points, which is economical to produce, while having high performance at radio frequencies. Increasingly high radio frequencies are being used as spectrum becomes scarce and demand for bandwidth increases. Furthermore, antenna systems are becoming increasingly sophisticated, often employing arrays of antenna elements to provide controlled beam shapes and/or MIMO (multiple input multiple output) transmission.
It is known to implement a radio transceiver having an array of antenna elements, which may be formed as copper areas printed on a dielectric film, for example a polyester film. A feed network is also printed on the dielectric film, to connect the antenna elements to the transmit and receive chains of the transceiver. A ground plate is provided, which underlies the film, and which provides a radio frequency ground plane for the antenna array and feed network. The ground plate has a top surface which underlies the film by a separation distance, which is arranged to be an appropriate distance to give the desired characteristic impedance for the feed network, in conjunction with the line width of the signal tracks of the feed network. The ground plate may have depressions provided under the antenna elements to improve radiation performance. The separation distance between the film and the ground plate is maintained by providing a dielectric spacer layer between the film and the ground plate, which may be composed of a foam material having low loss properties at radio frequencies. By using a thin film thickness in conjunction with a low loss dielectric spacer layer, the radio frequency loss of the feed network and antenna array is reduced. The film may be made of a material having a relatively high dielectric loss, but because it is thin in comparison with the dielectric spacer layer, the electric fields between ground plate and the feed network and the antenna elements pass through the low loss foam material for most of their length, and so the overall loss is low. An example of such a construction is given in UK patent application GB2296385, applied for by Northern Telecom Limited.
However, if it is desired to produce a compact design, for example using narrow tracks in the feed network, and for example for use at radio frequencies of 5 GHz and higher, then a small separation distance is required between the feed tracks and the ground plane in order to maintain the characteristic impedance of the tracks, and it may be difficult to manufacture and handle a sufficiently thin dielectric spacer layer made from a low loss foam material. It is undesirable to use a thicker film to support the feed network and antenna elements as this may result in a higher loss.
It is an object of the invention to mitigate the problems of the prior art.